diff --git a/doc/source/whatsnew/v2.2.0.rst b/doc/source/whatsnew/v2.2.0.rst
index ef59c86a21598..bf396b8124136 100644
--- a/doc/source/whatsnew/v2.2.0.rst
+++ b/doc/source/whatsnew/v2.2.0.rst
@@ -403,6 +403,7 @@ Other
 ^^^^^
 - Bug in :func:`cut` incorrectly allowing cutting of timezone-aware datetimes with timezone-naive bins (:issue:`54964`)
 - Bug in :meth:`DataFrame.apply` where passing ``raw=True`` ignored ``args`` passed to the applied function (:issue:`55009`)
+- Bug in Cython :meth:`StringHashTable._unique` used ephemeral repr values when UnicodeEncodeError was raised (:issue:`45929`)
 - Bug in rendering ``inf`` values inside a a :class:`DataFrame` with the ``use_inf_as_na`` option enabled (:issue:`55483`)
 - Bug in rendering a :class:`Series` with a :class:`MultiIndex` when one of the index level's names is 0 not having that name displayed (:issue:`55415`)
 -
diff --git a/pandas/_libs/hashtable_class_helper.pxi.in b/pandas/_libs/hashtable_class_helper.pxi.in
index 1cf5d734705af..79a0dfc6d723d 100644
--- a/pandas/_libs/hashtable_class_helper.pxi.in
+++ b/pandas/_libs/hashtable_class_helper.pxi.in
@@ -1128,6 +1128,7 @@ cdef class StringHashTable(HashTable):
         use_na_value = na_value is not None
 
         # assign pointers and pre-filter out missing (if ignore_na)
+        keep_rval_refs = []
         vecs = <const char **>malloc(n * sizeof(char *))
         for i in range(n):
             val = values[i]
@@ -1144,7 +1145,9 @@ cdef class StringHashTable(HashTable):
                 try:
                     v = get_c_string(<str>val)
                 except UnicodeEncodeError:
-                    v = get_c_string(<str>repr(val))
+                    rval = <str>repr(val)
+                    keep_rval_refs.append(rval)
+                    v = get_c_string(rval)
                 vecs[i] = v
 
         # compute